In this type of typical exposure apparatus, light beam emitting from a light source forms a secondary light source as a substantive surface illuminant comprised of many light sources via a fly's eye lens (or micro lens array) as an optical integrator. The light beam from the secondary light source is converged by a condenser lens, then is superimposed and illuminates a mask where a predetermined pattern is formed. The light transmitted through the pattern of the mask form images on a photosensitive substrate via a projection optical system. In this way, the mask pattern is projected and exposed (transferred) on the photosensitive substrate.
In the exposure apparatus, a light detecting apparatus for detecting the exposure light quantity (exposure dose) for the photosensitive substrate is provided. This light detecting apparatus for an exposure monitor extracts a part of the illumination light (exposure light) from the optical path, and detects the intensity of the extracted light (e.g. patent document 1). By adjusting the emission output of the light source according to the change of the light intensity detected by the light detecting apparatus, the exposure light quantity for the photosensitive substrate is controlled so as to be substantially constant during exposure.
Patent Document 1: Japanese Patent Application Laid-Open No. H8-203803